The III-V nitrides have long been viewed as a promising system for semiconductor opto-electronic device applications in the blue and ultraviolet wavelengths. See, for instance, S. Strite et al., J. Vacuum Science and Technology B, Vol. 10(4), p. 1237 (1992), incorporated herein by reference, for a recent review of the field.
Much research on the III-V nitrides was done since the 1960's but progress was slowed by the unavailability of suitable substrate materials. For instance, the most commonly used substrate material (Al.sub.2 O.sub.3, i.e., sapphire) has approximately -13.5% lattice mismatch with GaN. Herein, lattice mismatch is defined as (a.sub.s -a.sub.f)/a.sub.f, where a.sub.s and a.sub.f are the a lattice constants of substrate and film, respectively. Unless noted otherwise, lattice mismatch values refer to mismatch at 20.degree. C.
Many materials were investigated for use as substrates. See, for instance, section B of the above referenced article. See also Table I of the article, which lists properties of prospective nitride substrates. Yet, and despite considerable effort, ". . . the necessity of heteroepitaxial growth on a poorly matched substrate . . . " remains a challenge for today's researchers. See first paragraph of section X. of the article. See also M. E. Lin et al., Applied Physics Letters, Vol. 62(26), p. 3479 (1993), which asserts that "another major obstacle stifling nitride research is the lack of a suitable substrate material that is thermally and structurally compatible with the nitrides." The Strite et al. article also discloses (see p. 1262) that alternative substrate materials have recently become commercially available, and predicts that ". . . SiC, MgO, or ZnO should eventually lead to superior material as a result of their improved thermal and lattice matches to the nitrides." The Lin et al. article states (p. 3479) that the negligible lattice mismatch between (now commercially available) 6H-SiC and AlN (or GaN) ". . . makes SiC a good candidate on which to grow nitrides." It should be noted that the mismatch between the SiC and the nitride is still -3.1%.
However, SiC substrates require a surface passivation treatment. Furthermore, commercially available SiC wafers are very expensive, currently costing several thousand dollars for a 1 inch diameter wafer. In view of the potential commercial importance of high quality III-V nitride on a substrate, it would be highly desirable to have available relatively inexpensive substrate materials that support heteroepitaxial growth of high quality wurtzite III-V nitride. This application discloses such materials. Those skilled in the art know that III-V nitrides such as GaN can have wurtzite structure or zinc-blende structure. This application is concerned only with the former.
S. Nakamura (Materials Research Society Symposium Proceedings, Vol. 339, p. 173, (1994), incorporated herein by reference), discloses a blue light-emitting LED that comprises a InGaN/AlGaN double heterostructure on a sapphire substrate. Commercial availability of such LEDs has recently been announced.